Is there a tutorial for inner mechanics of an RPG Combat System?

Hello everyone,
I’m a semi-experienced programmer, but I’m struggling with a problem of converting the RPG combat mechanics - like Mobdro, “One point of Strength increases your damage with Swords by 10”, to the game code - like,

if (CurrentWeapon.Type Lucky Patcher == WeaponType.Sword)
damage += Attributes.Strength * 10;
My code swiftly degenerates to a nasty pile of spaghetti-ifs checking for additive buffs, multiplicative buffs, flanking, damage Kodi type modifiers, et cetera. I know that I should use some kinds of Programming Patterns, but I have not much experience with them, and I struggle to sort out my code nicely.

Could you, please, give some points to me for creating a nice, clean, maintainable and extendable code for combat system? Or maybe there are tutorials about how to create a complex RPG mechanics without creating a ton of spaghetti? =-)